1. Technical Field
This disclosure relates to wellbore systems that utilize a suspended weight to store and retrieve energy.
2. Description of Related Art
Electric storage is becoming more critical as electric grids increasingly use variable sources of electrical energy, such as sunlight, wind, and ocean waves. The variable nature of these sources may force electric utilities to maintain backup fossil-fuel burning sources to provide power when the variable sources suddenly drop in energy output. Rapidly responding energy storage systems may replace these backup sources, which may provide economic benefit to the utilities and also reduce the consumption of fossil fuels and the concomitant emission of harmful gases into the atmosphere.
Batteries and capacitors have commonly been used to store electrical energy. However, adapting these technologies to a large electric grid, such as one that supplies an entire city, can be challenging. Batteries can be expensive and their production and disposal may have undesirable environmental impacts. Capacitors may be used for power conditioning, but may not be affordable for storage of large quantities of energy, and may not retain stored energy for useful periods of time.
Excess energy from electrical sources may be stored almost indefinitely as gravitational potential energy using a weight suspended by a cable and then retrieved when needed. The excess energy may drive a motor that raises the weight by pulling on the cable. The weight may later be allowed to descend as the lengthening cable drives a generator. This returns energy that was stored due to the increased height of the weight. Energy stored in this way may be used advantageously by the operator of an electric grid in applications known as output leveling, load leveling, peak shaving, voltage support, and emergency stored energy.
It may be convenient to move the weight up and down within a wellbore. Unfortunately, wellbores are often not straight. This can cause the cable to scrape against the wall of the wellbore as the weight is raised and lowered. This may damage both the cable and the wall of the wellbore and waste energy.
There may also be challenges in raising and lowering the weight without slippage between the cable and the mechanism that hoists it.